Speed governors are frequently utilized with small commercial or industrial engines to maintain a substantially constant engine speed during varying load conditions. One common type of system is associated with the cam shaft of the engine and consists of a plurality of flyweights carried by a flyweight carrier mounted on the cam shaft. The flyweights are pivoted to the carrier in a manner such that centrifugal force acts to move the flyweights away from the axis of the cam shaft. Each flyweight includes an abutment or finger, which bears against the end of a spool that is mounted coaxially with the cam shaft. The spool is secured to the shaft in a manner such that it rotates with the shaft, but can slide axially with respect to the shaft.
Axial movement of the spool under the action of the flyweights is resisted by a fork which straddles the cam shaft and is spring biased into engagement with the opposite end of the spool. The fork is secured to a rotatable rod or shaft which is operably connected to the throttle of the engine. With this arrangement, the biasing force of the spring acts in opposition to the centrifugal force on the flyweights to thereby maintain the engine speed relatively constant regardless of the load on the engine.
With the governing system as described above, the flyweights and spool both rotate with the cam shaft, so that there is no relative rotation between the members. However, as the engine speed changes, the flyweights tend to move radially relative to the spool as the centrifugal force exceeds the force of the biasing spring. This relative radial motion of the fly-weights, small as it is, is resisted by frictional forces. The spool does not move, therefore, until the difference between the centrifugal force and the biasing spring force exceeds the frictional forces.
It is desirable to reduce the frictional forces that resist movement of the flyweights and the spool, because they add and substract from the centrifugal force, depending on whether the flyweights are moving radially in or out. These frictional forces cause the governing system both to "hunt" up and down in speed and to hold an erratic speed.